We studied carbon-nanotube quantum dot embedded in a superconducting-quantum-interference-device loop in order to investigate the competition of strong electron correlations with a proximity effect (Figure a). Depending on whether local pairing or local magnetism prevails, a superconducting quantum dot exhibits a positive or a negative supercurrent, referred to as a 0 or π Josephson junction, respectively.
Using metallic contact on a suspended single wall nanotube (Figure b), we also reported on cotunneling spectroscopy magneto-conductance measurements of ultraclean carbon nanotube quantum dots in the SU(4) Kondo regime with strong spin-orbit coupling.
We demonstrated the effect of single-electron tunneling (SET) through a carbon nanotube quantum dot on its nanomechanical motion (Figure c). We found that the frequency response and the dissipation of the nanoelectromechanical system to SET strongly depends on the electronic environment of the quantum dot, in particular, on the total dot capacitance and the tunnel coupling to the metal contacts.
We also investigated Kondo effects. As an example, we presented the first quantitative experimental evidence for the underscreened Kondo effect, an incomplete compensation of a quantized magnetic moment by conduction electrons, as originally proposed by Nozières and Blandin. The device, a single molecule transistor obtained by electromigration of C60 molecules into gold nanogaps and operated in a dilution fridge (Figure d), was tuned into its spin S=1 regime.
We demonstrate the possibility of a quantum phase transitions in a single-molecule quantum dot, where a gate voltage induces a crossing of two different types of electron spin state (singlet and triplet) at zero magnetic field.
A SQUID with single-walled carbon nanotube (CNT) Josephson junctions is presented. The CNTs behave as quantum dots and allow some control on the SQUID phase using electrostaic gates.